Memory is a key cognitive function involving the storage and/or retrieval by the brain of information received from past experiences. Learning, also referred to as conditioning, is the process by which new information is acquired and stored by the nervous system to form a memory. In patients with dementia, the cognitive pathways for learning and/or memory are impaired, such that the patient fails to learn or effectively form new memories or recall old ones. The number of individuals exhibiting dementia is rising rapidly, and the rate of rise is expected to increase as the general population continues to age and life expectancy continues to lengthen. Patients with dementia require increasingly costly and intensive caregiving as their symptoms worsen. As such, medical interventions that delay institutionalization would help reduce the demands on healthcare systems, in addition to alleviating the sufferings of the subject with the dementia.
The development of profound dementia is characteristic of several amyloidogenic disorders noted for the accumulation of amyloid protein deposits in the brain tissue of affected subjects, including Down's syndrome, cerebral amyloid angiopathy, vascular dementias, and Alzheimer's disease (AD). AD is a progressive disease resulting in senile dementia. Broadly speaking, the disease falls into two categories: late onset, which occurs in old age (65+ years) and early onset, which develops well before the senile period, i.e., between 35 and 60 years.
Neurodegeneration is associated with amyloidogenic disorders and other dementia disorders such that the cognitive symptoms progressively worsen with age. The diagnosis of an amyloidogenic disorder can usually only be confirmed by the distinctive cellular pathology that is evident on post-mortem examination of the brain. The histopathology consists of at least one of three principal features including the presence of neurofibrillary tangles (NT), the diffuse loss of synapses and neurons in central nervous system tissues, and the presence of amyloid plaques (also called senile plaques). See generally Selkoe, TINS 16:403 (1993); Hardy et al., WO 92/13069; Selkoe, J. Neuropathol. Exp. Neurol. 53:438 (1994); Duff et al., Nature 373:476 (1995); Games et al., Nature 373:523 (1995).
The principal constituent of the plaques is a peptide termed Aβ or β-amyloid peptide. Aβ peptide is an approximately 4-kDa internal fragment of 39-43 amino acids of a larger transmembrane glycoprotein named protein termed amyloid precursor protein (APP). As a result of proteolytic processing of APP by different secretase enzymes, Aβ is primarily found in both a short form, 40 amino acids in length, and a long form, ranging from 4243 amino acids in length. Part of the hydrophobic transmembrane domain of APP is found at the carboxy end of Aβ, and may account for the ability of Aβ to aggregate into plaques, particularly in the case of the long form. Accumulation of amyloid plaques in the brain eventually leads to neuronal cell death. The physical symptoms associated with this type of neural deterioration characterize AD.
Several mutations within the APP protein have been correlated with the presence of AD. See, e.g., Goate et al., Nature 349:704 (1991) (valine717 to isoleucine); Chartier Harlan et al., Nature 353:844 (1991) (valine717 to glycine); Murrell et al., Science 254:97 (1991) (valine717 to phenylalanine); Mullan et al., Nature Genet. 1:345 (1992) (a double mutation changing lysine595-methionine596 to asparagine595-leucine596). Such mutations are thought to cause AD by increased or altered processing of APP to Aβ, particularly processing of APP to increased amounts of the long form of Aβ (i.e., Aβ 1-42 and Aβ1-43). Mutations in other genes, such as the presenilin genes, PS1 and PS2, are thought indirectly to affect processing of APP to generate increased amounts of long form Aβ (see Hardy, TINS 20: 154 (1997)).
Mouse models have been used successfully to determine the significance of amyloid plaques in AD (Games et al., supra, Johnson-Wood et al., Proc. Natl. Acad. Sci. USA 94:1550 (1997)). In particular, when PDAPP transgenic mice, (which express a mutant form of human APP and develop AD pathology at a young age), are injected with the long form of Aβ, they display both a decrease in the progression of AD pathology and an increase in antibody titers to the Aβ peptide (Schenk et al., Nature 400, 173 (1999)). The above findings implicate Aβ, particularly in its long form, as a causative element in AD.
Aβ peptide can exist in solution and can be detected in the central nervous system (CNS) (e.g., in cerebral spinal fluid (CSF)) and plasma. Under certain conditions, soluble Aβ is transformed into fibrillary, toxic, β-sheet forms found in neuritic plaques and cerebral blood vessels of patients with AD. Several treatments have been developed which attempt to prevent the formation of Aβ peptide, for example, the use of chemical inhibitors to prevent the cleavage of APP. Immunotherapeutic treatments have also been investigated as a means to reduce the density and size of existing plaques. These strategies include passive immunization with various anti-Aβ antibodies that induce clearance of amyloid deposits, as well as active immunization with soluble forms of Aβ peptide to promote a humoral response that includes generation of anti-Aβ antibodies and cellular clearance of the deposits. Both active and passive immunization have been tested as in mouse models of AD. In PDAPP mice, immunization with Aβ was shown to prevent the development of plaque formation, neuritic dystrophy and astrogliosis. Treatment of older animals also markedly reduced the extent and progression of these AD-like neuropathologies. Schenk et al., supra. Aβ immunization was also shown to reduce plaques and behavioral impairment in the TgCRND8 murine model of AD. Janus et al. (2000) Nature 408:979-982. Aβ immunization also improved cognitive performance and reduced amyloid burden in Tg 2576 APP/PS1 mutant mice. Morgan et al. (2000) Nature 408:982-985. Passive immunization of PDAPP transgenic mice has also been investigated. It was found, for example, that peripherally administered antibodies enter the central nervous system (CNS) and induced plaque clearance in vivo. Bard et al. (2000) Nat. Med. 6:916-919. The antibodies were further shown to induce Fc receptor-mediated phagocytosis in ex vivo assay. Antibodies specific for the N-terminus of Aβ42 have been demonstrated to be particularly effective in reducing plaque both ex vivo and in vivo. See U.S. Pat. No. 6,761,888 and Bard et al. (2003) Proc. Natl. Acad. Sci. USA 100:2023-2028. Antibodies specific for the mid-region of Aβ42 also showed efficacy. See U.S. Pat. No. 6,761,888.
Two mechanisms are proposed for effective plaque clearance by immunotherapeutics, i.e., central degradation and peripheral degradation. The central degradation mechanism relies on antibodies being able to cross the blood-brain barrier, bind to plaques, and induce clearance of pre-existing plaques. Clearance has been shown to be promoted through an Fc-receptor-mediated phagocytosis (Bard, et al. (2000) Nat. Med. 6:916-19). The peripheral degradation mechanism of Aβ clearance relies on a disruption of the dynamic equilibrium of Aβ between brain, CSF, and plasma by anti-Aβ antibodies, leading to transport of Aβ from one compartment to another. Centrally derived Aβ is transported into the CSF and the plasma where it is degraded. Recent studies have concluded that soluble and unbound Aβ are involved in the memory impairment associated with AD, even without reduction in amyloid deposition in the brain. Further studies are needed to determine the action and/or interplay of these pathways for Aβ clearance (Dodel, et al., The Lancet, 2003, 2:215)
While the majority of treatments to date have been aimed at reducing amyloid plaque buildup, it has been recently noted that certain cognitive impairments (e.g. hippocampal-dependent conditioning defects) associated with amyloidogenic disorders begin to appear before amyloid deposits and gross neuropathology are evident (Dineley et al., J. Biol. Chem., 2002, 227: 22768). Furthermore, while the pathogenic role of amyloid peptide aggregated into plaques has been known for many years, the severity of dementia or cognitive deficits is only somewhat correlated with the density of plaques whereas a significant correlation exists with the levels of soluble Aβ. (see, e.g., McLean et al., Ann Neurol, 46:860-866 (1999). Some studies have shown or suggested that soluble Aβ oligomers are implicated in synaptotoxicity and memory impairment in APP transgenic mice due to mechanisms including increased oxidative stress and induction of programmed cell death. (See, e.g., Lambert, et al., (1998), PNAS, 95: 6448-53; Naslund et al., (2000), JAMA, 283: 1571; Mucke et al., J. Neurosci, 20:4050-4058 (2000); Morgan et al., Nature, 408:982-985 (2000); Dodart et al., Nat Neurosci, 5:452-457 (2002); Selkoe et al., (2002), Science, 298: 789-91; Walsh et al., Nature, 416:535-539 (2002)). These results indicate that neurodegeneration may begin prior to, and is not solely the result of, amyloid deposition. Accordingly, there exists the need for new therapies and reagents for the treatment of AD, in particular, therapies and reagents capable of effecting a therapeutic benefit via intervention with various mechanisms of Aβ-induced neurotoxicity.